Include kwant's examples directly in the tutorials instead of importing them

This tutorial imports from scripts which isn't in the package reference. I think this makes the tutorial a bit cryptic. An alternative is to include the code directly in the tutorial in a toggle-box. This way anyone that wants to understand what it is can read the code directly without going into the source code, especially since it is not in the package reference. A toggle-box also allows to hide it so it does not distract from the main story of the tutorial.